Hematopoietic stem cells (HSC) in the bone marrow provide, through a tightly regulated process of differentiation and proliferation, lifelong production of blood cells. With age however, striking changes occur in the hematopoietic system. One of these changes involves lymphopoiesis. Here, we focus on age-related changes in B-cell development, which is characterized by a loss of early B-cell precursors. The severe age-related depletion of early B-cell precursors, and especially of pre-B cells, is not accompanied by a decline in the peripheral B cell pool, although it has been proposed that a loss of follicular B cells is compensated for by an increase in marginal zone B cells in aged mice. A coherent explanation for age-related changes in B-cell development and in the peripheral B cell pool was lacking thus far. We show here that with age, an alternative B-cell development pathway emerges where an identifiable pre-B cell phase is absent. This pathway originates from a population of Iin-sca1? (LSK-) cells in the bone marrow. The age-related emergence of this alternative B-cell development pathway may provide a unifying explanation for the thus far almost inexplicable patterns of depletion of B-cell precursor populations in aged mice, together with maintenance of mature B cells and changes in antibody repertoire. Here, we propose to analyze this pathway functionally and phenotypically, and to delineate its biological significance. The specific aims are: Specific aim 1: To elucidate the differentiation pathway from LSK cells to B cells;Specific aim 2: To investigate the mechanism of LSK- generation and differentiation;Specific aim 3: To examine the biological significance of the LSK- pathway.